Luminescence of PbCl2 and PbBr2 single crystals II. Luminescence and EPR of uv irradiated crystals

Abstract

PbClz and PbBr, single crystals show a red luminescence under uv excitation at temperatures below 200°K. Furthermore, PbCl* shows a yellow emission at temperatures below 40°K. The centers responsible for these emissions have been investigated by EPR measurements. These measurements indicate that due to uv .. . n-radlatlon Pb;,, (PbL& and lead colloids are created. In PbC12, a hole center, possibly Cl& is created at temperatures below 50°K. Both the concentration of the Pb;, and the (Pb& centers in PbClz depends on the irradiation intensity; the concentration of the pair-centers increases with time after the irradiation is stopped. A similar dependence has been observed on PbBrz crystals cleaved and mounted in the dark, but in this case the influence of the irradiation appears to be smaller. The defects responsible for the red and yellow luminescence could be identified on the basis of temperature dependence measurements. The red luminescence of both halides is ascribed to excitation and decay of Pb;, or (Pb& centers, and the yellow luminescence of PbCI* is associated with Cl,& centers.

Self-trapped states and related luminescence inPbCl2crystals

“…In the early stage around 1970, the localized states induced by ultraviolet (UV) light irradiation at 80 K were observed around g ≈ 2 (Refs. 4,5,6). Figure 1 shows ESR signals photoinduced at 80 K; a set of five resonances around g ≈ 2 was named as "A signal" 5 and is enlarged in the inset.…”

“…The intense photoluminescence (PL) at low temperatures below 10 K was classified to UV-PL band at 3.8 eV, blue-PL band at 2.8 eV, and blue-green (BG) PL band at 2.5 eV. 5,13,14 The UV-PL and the blue-PL bands are mainly induced under one-photon excitation into the exciton band, and the BG-PL band is dominantly induced under excitation into the energy region higher than the lowest exciton. 39 As the origins of the UV-PL band at 3.76 eV and the blue-PL band at 2.88 eV, the self-trapped excitons (STE's) with the configuration of (Pb + + hole) were proposed, and the origin of the BG-PL band was attributed to the STE's of (Pb 2 3+ + hole).…”

“…2 is the B signal named in Ref. 5; it is far weaker than the C signal and hardly depends on rotation angles. From now on, we focus on the main signals of the A, the C, and the satellite signals.…”

“…The intense line at 326 mT was induced by x-ray irradiation at 10 K and was named as C signal in Ref. 5. The small signal at 336 mT in the inset of Fig.…”

Luminescence Decay in NaI:Pb

“…This seems to be supported by the similarity between the characteristics of this band and the excitonic emission band of the PbI, single crystals [12], i.e. the fast radiative lifetime, the strong temperature dependence of the intensity, the low Stokes loss of the luminescence, the shape of the luminescence spectrum which is a mirror-like image of the excitation spectrum, and finally the fact that under irradiation in the A absorption band region at room temperature the precipitated phase of Pb in NaI matrix presents photodecomposition processes [ll J similar to those observed in PbI, by irradiation in the 3P1 cation excitonic absorption band [13].…”